Rechargeable batteries are commonly used to power portable electronic devices, such as laptop computers, PDAs, digital cameras and MP3 players. Many of those portable electronic devices include circuitry for charging their batteries whenever the devices are connected to external power sources such as a wall adapter, USB, Firewire, and Ethernet. For example, the USB itself can be used to directly power the devices and charge their batteries. According to the USB specification, USB hosts, or USB powered hubs, provide as much as 500 mA from their nominal 5V supply. The USB is known as a current constrained source.
FIG. 1 is a schematic diagram showing circuit topology for providing power to a load and charging a battery, incorporated into a portable USB device. As shown in FIG. 1, a USB linear charger 2 generally provides current constrained power directly to a battery 4 to which a system load 6 is tied in parallel with battery 4. This topology maintains the USB current constraint but sacrifices efficiency in that there is a substantial voltage drop from a USB input voltage to a battery voltage. Since load 6 is tied directly to battery 4, if the battery voltage is very low or battery 4 is dead, there will not be enough voltage applied to load 6 to run an application. The voltage input to system load 6 is the battery voltage and the current drawn by system load 6 is equal to the power requirement of load 6 divided by the battery voltage. This is true even if there is external power applied to load 6 and battery 4 because the battery dictates the voltage applied to load 6. When battery 4 is fully discharged, several minutes of charging may be required before any load can be connected to the battery. Moreover, many battery or handheld applications have peak current that can exceed the 500 mA USB specification. Input current from the current constrained source to USB linear charger 2 needs to be controlled properly when peak current of load 6 exceeds the USB specification.
The subject matter described herein addresses, but is not limited to, the above shortcomings.